Sewage digestion



1937. A. J. FISCHER ET AL 2,091,631

SEWAGE DIGESTION Original 'Filed May 31, 1934 7 Sheets-Sheet 1 Outlawmokmw 5) (I'ttorneg Aug. 31, 1937. A. J. FISCHER ET AL SEWAGE DIGESTION"7 Sheets-Sheet 2 Original Filed May 31, 1934 1937- "A. J. FISVCHER ETAL 2,091,631

SEWAGE DIGESTION Original Filed May 51, 1934 7 Sheets-Sheet 3 g- 1937-A. J. FISCHER ET AL 2,091,631

SEWAGE DIGESTION Original Filed May 31, 1934 7 Sheets-Sheet 4 GttomegAug. 31, 1937. A. J. FISCHER ET AL- SEWAGE DIGESTION Original Filed May31, 1934 "7 Sheets- Sheet 5 FIG 3 n 1: en tors 4077/0 d. W509i attornegI A. J. FISCHER ET AL 2,091,631

SEWAGE DIGESTION Aug. 31, 1937.

Original Filed May 51, 1934 7 Sheets-Sheef s lung L (Yttorncg Aug. 31,193 7.

J. FISCHER ET AL SEWAGE *DIGES'I'ION Original Filed May 51, 1934 7Sheets-Sheet 7 Patented Aug. 31, 1937 UNITED STATES FFICE Lund, Seaford,N. Y, assignors Company, inc, New York, N.

of Delaware Original application to The Dorr a corporation May 31, 1934,Serial No.

728,376. Divided and this application January 28, 1935, Serial No. 3,726l 9 Claims.

In the treatment of sewage and analogous polluted waters, according tomodern practice, it is quite common to effect an early separatingas bysettling and withdrawal of the suspended solids of the sewage in theform of a sludge and to subject the withdrawn sewage sludge to what isknown as sewage sludge digestion. Sometimes in this class of work thesewage sludge is obtained in a condition which is known as raw oruntreated sludge.

According to other procedures either the sewage is chemically treated orthe sludge may be treated whereby there may be obtained what may betermed a chemically treated sludge. The chemical treatment may have beenprovided to facilitate 5 the precipitation for the ultimate obtaining ofthe solids as sewage sludge. Also according to certain procedures thesewage or the sewage sludge is subjected to aeration which is employedas for example to facilitate the precipitation of the sludge 0 solidscalled activated sludge or in order to obtain a sewage sludge havingcertain characteristics which facilitate the process as a whole.

Raw or untreated sludge often has acid characteristics. Where the sewageis aerated the sewage 25 sludge obtained may have an acid characteristicparticularly where the sludge is stored prior to digestion.

When sewage sludge is digested the acid characteristic has to beovercome and the sludge takes 0 on an alkaline characteristic which isdesirable throughout the entire digesting period. This alkalinecharacteristic may be the result of.

anaerobic bacterial action which results in digestion which is dulyinitiated and which is main 35 tained. This digestion is the conversionby virtue of bacterial activity of certain of the sewage solids intosome other physical form such as a liquid or a gas. Some sewage solidsare digestible while others are not. Digestible sewage solids are Nlargely putrescible, so theirconversion by digestion thus eliminatesfrom digested sewage sludge.

. The present invention is concerned with apparatus useful in connectionwith the digesting of 5 sewage sludges or mixtures thereof no matter howobtained, for example, in sludge digesters in which the sludge treatmentcarried out therein is by a method in which there is an anaerobicdigestion of the sewagesludge as the result of biological or 50bacteriological activity during which certain bothersome sewage solidsundergo decomposition and distintegration, such digestion is accompaniedwith the production of a combustible gas containing methane as asubstantial con- 55 stituent thereof.

their odoriferous nuisancev The present invention primarily relates toapparatus or instrumentalities usable for or in connection with thebiological digestion of sewage sludge and its analogues and one of theobjects thereof is to construct in commercial practical form anapparatus or instrumentality useful in the treating of the sludge and sothat during said treatment the gas developed and given off as theprocess progresses can be collected and handled.

The apparatus which is the subject of the present invention embodiesagitating or mixing devices which will aid mixing conducive to theseeding action referred to herein and which hastens the production ofthe methane gas due to the bacterial action which takes place as. thesludge digestion process goes on.

The invention according to one aspect there- .of relates to theproviding for use in connection with a sewage sludge digester tank, of avertically yieldable or vertically movable structure or unitary topassembly 4 comprising (a) a sheet metal gas holding type of top withdepending marginal flange or peripheral wall; (b) a bracing and guidingmeans providing a centrally located vertically extending strut rigidlyconnected and depending from the sheet metal top and serving if desiredas a vertically movable center guide element, and tie rods or connectingmembers extending from the marginal portion of the sheet metal topdownwardly and inwardly tothe lower end portion of the center strut; (a)one or more ofi-center sludge stirring devices each extending downwardlyinto and through an inte mediate opening provided in a section of thesheet metal top member between the center strut and the marginal flange,and essentially comprising a vertically extending driven rotatable shaftcarrying one or more sludge impelling or stirring elements, 2. basemember normallyseated on the sheet metal top and having a gas-tight sealthrough which the shaft extends and serving as a closure cap for thecorresponding intermediate opening; ((1) a steady bearing constructionat and for the lower end of each shaft; and (e) an arm constructionproviding an arm for a co respond! ing sludge stirring device, each saidarm extending outwardly, that is, horizontally from the lower endportion of the central strut, carrying at the outer end thereof a fixedmember of the steady (bearing construction, deriving support from thestrut because of being directly connected to the lower end portion ofthe latter, and preferably supported against downward deflection of theouter end thereof by a connecting brace extendingupwardly and inwardlyfrom the outer end one is able to portion of the arm to central strut.

upper portion of the The unitary top assembly is preferably made so asto permit ready removal and ready replacement of a sludge stirringdevice and therefore a sheet metal top is employed which is preferablyprovided with a depending intermediate flange along the marginal definesof an opening intermediate the central to is constructed so as to sealrealized as by a and marginal wall portion of the top. The base memberor cap referred provide a suitable gas gas sealing packing through whichthe stirring device shaft extends and this base or cap member operatingparts and preferably carries the shaft is detachably secured to thesheetmetal top thus permitting the sludge stirring device to be lifted as awhole from its normal position in respect to the sheet metal top. In

furtherance of a practical construction wherein the feature of readyremoval and replacement may be realized together with the attainment ofsuitable guiding for the lower end of the stirring device shaft, thesteady for, at portion of the shaft and and on the lower end liftabletherewith and inverted cone shaped or a cone shaped section fordirectingand for aiding in the proper seating of the one member on theother as the sludge stirring device is lowered into operative position.

Further aspects relate to the simplifying of the construction of thedigester; to the reducing of the cost of its operation; and tothedevising of means for preventing troublesome scum formation therein.

This application is a division of a parent application Serial No.728,376 filed May 31, 1934, and which "eventuated into Patent No.1,989,589

granted January 29, 1935. cation it was pointed can be depended upon forfresh sewage sludge with In the parent appliout that digester liquorseeding or inoculating anaerobic bacteria to initiate and to stimulatethe bacteriological digestion thereof to a greater degree than candigester solids, when considered on a dry solids-content basis. 1digested sludge solids to Formerly it was believed that the ratio of theraw sludge solids was the determining factor in calculating the volumeof seeding 5 aforementioned discovery material required. With the inmind'one aspect of the apparatus of this inventionis to eflectonly suchmovements within the body of digesting sludge as will permit theheaviersludge solids to exist within a limited horizontal zone whileotherwise the movement of the is circulated throughout the entirerestricted and liquor is relatively unmass in the digester of which theapparatus of this invention cal element.

realize the constitutes an important mechani- By the apparatus of thisinvention selective and diflerential' circulation of digester liquor anddigester solids.

According to one and in certain respects preferred arrangement (orarrangements) of apparatus for realizing the basic aspects of theinvention there is employed a primary digester (or primary digesters)having a vertically yieldable or vertically movable top or gas holderconstruction. In any event in of the invention there are holder or topmember or realizing certain aspects employed with the gas cover of theprimary of digesting sludge ensures tion revolve about this constructionof the gasholder and particularly about the construction of theyieldable top of the primary digester so that it will have the requiredmovement. hlkaline condition essential for the carrying out of thesewage digestion by anaerobic bacteria has been once established thatcondition will not be upset by incoming fresh sludge, even though theincoming fresh sludge may have-an acid characteristic, because theimmediate intimate mixing of the incoming sludge through the alkalinebody the preponderance and prevalence of the alkaline characteristic andanaerobic bacterial activity.

The invention possesses other objects and features of advantage, some ofwhich with theforegoing will be set forth in the following description.

In the following description and in the claims parts will be identifiedby specific names for convenience but they are intended to be as genericin their application to similar parts as the art will permit. In theaccompanying drawings which constitute a part of this specificationthere have been illustrated the best and most characteristic embodimentsof the invention known to the undersigned but such embodiments are to beregarded as typical only of many possible embodiments and the inventionis not to be limited thereto.

Insaid drawings Fig. 1 is a diagrammatic of a sewage system embodyingtwo sedimentation tanks either or both of which can be relied upon toreceive a stream of sewage and from either or both of which at willsewage sludge derived as a corresponding to incoming sewage sludge asand when the latter is digester.

This figure yieldable peller element in submergence is rotated about anupwardly extending axis of a vertical shaft carrying the same.

Figures 3 and 4 are tional and plan views of a multiple digester.

Fig. 5 is a vertical sectional view partially broken away showing therelative arrangement and position ofcertainrparts of the primarydigester in respect to; certain other parts of the secondary digesterandmore particularly the relative position of certain overflowconstructions provided for the respectively vertically secprimarydigester in respect to 75 I certain overflow constructions provided forthe secondary digester.

Fig. 6 is a perspective view illustrative of the overflow arrangementsof Fig. 5. In Figs. 3 to 6 inclusive the overflow arrangements for thesecondary digester are shown functioning in a horizonal path 180 degreesfrom the position which the same parts occupy in Fig. 1.

Fig. 7 is a vertical sectional view taken as on the plane indicated bythe line ll-l of Fig. 5

looking in the direction of the arrows, this Fig. 7

being a vertical sectional view showing the overflow arrangement for theprimary digester in respect to the inlet end of a pipe or conduitleading to the secondary digester.

Fig. 8 is a vertical sectional view taken as on the plane indicated bythe line d--t of Fig. 5 looking in the direction of the arrows, thisFig. 8 being a vertical sectional view showing certain overflowarrangements for the secondary digester in respect to the inlet end of apipe or conduit for conducting: the relatively clear effluent liquid toa place for the disposition of the same.

Fig. 9 is a perspective view of the vertically movable top member or gasholder used in the primary digester but it will be understood that themain features of this gas holder as embodied therein may also beembodied in the gas holder employed in the secondary digester. There isnot included in Fig. 9 the mixing devices carried by the gas holder.

Fig. 10 is a view the mixing device and the manner in which it isapplied to the gas holder of Fig. 9.

Fig. 11 is a partial sectional view taken as on the plane indicated bythe broken lines ll-i.l of Fig. 3 looking in the direction of thearrows.

Fig. 12 is a vertical sectional view taken as on the plane indicated bythe line l2-l2 of Fig. 11 looking in the direction of the arrows.

Fig. 13 is a plan view of an impeller element used in the mixing deviceof Fig. 10.

Figs. 5 to 13 inclusive are shown at a much larger scale than the sameparts are shown in Figs. 3 is at a smaller scale than any of the otherfigures.

Fig. 14 shows a primary and secondary digester construction thatfunctions practically the same as the digesters shown in Fig. 3 exceptthat in Fig. 14 the gas holder of the primary digester is shown as afloating cover arrangement that is provided with a pontoon constructionthat defines the peripheral wall thereof. In Fig. 3 each of the gasholders is shown as resting on stopsto wit, a position below that whichthey occupy in normal operation.

Fig. 15 shows a construction wherein primary and secondary digesters areemployed that function much the same as the digesters in the ar'rangement of Figs. 3 and 14 but in the primary digester of Fig. 15 thenormal position for the yicldable top is that which it occupies whenresting upon the stops. In other words the yieldable top of the primarydigester of this figure leaves the stops only due to a decidedlyabnormal condition existing within the digester and in which instance itis free to yield or move upwardly.

Reference will now be made to the drawings in detail.

Like reference characters indicate parts functioning the same orsubstantially alike wherever they appear.

In the arrangement indicated in Fig. 1 the incoming sewage supply isdesignated by S8. The

partially in section showingand 4 and it will also be noted that Fi 1sewage from this supply is passed at will as along the fiath l to theclarifier, sedimentation tank, or sedimentation basin 2 or as along thepath 3 to the clarifier d. Settled sludge is withdrawn at will from oneor both clariflers as along the path 5 or the path 6 as the case maybeand is delivered intermittently by means of sludge pumps l and/or 3ultimately along the path 9 that terminates in feed pipe M5 to and intothe primary' digester iii) constituting part of a mul tiple digestionsystem that also comprises the secondary digester tank Ml arranged toreceive intermittently from the primary digester sludge which isundergoing digestion and in an amount which is equal to the amount asand when fed into the primary digester for treatment. This tank ill andits equivalents, while herein are referred to as secondary digesters,are primarily for carrying out sedimentation and secondarily forcarrying out digestion.

Each of these digesters comprises a digester tank and an associated gasholder or gas collector. The tank of the primary digester is designatedby M2 and its associated gas holder or collector by M3. The tank for thesecondary digester is designated by Hi l while its associated gas holderor collector is designated by M5.

As previously indicated piping or other passage is provided wherebypartially digested sludge can pass from the primary digester to thesecondary digester as is required incident to the supplying of theincoming sludge to the primary digester. Piping is also provided wherebythe gas receiving spaces within the two digesters are in communicationwith each other during all normal periods of operation. These pipingarrangementswill later be described more in detail.

Also as previously indicated the digester arrangement of the system ofFig. 1, to wit, the

arrangement. of the primary and secondary digesters thereof asconstructed and connected, is essentially that of the arrangements ofFigs. 3 to 15 inclusive. The early work in connection with the actualcarrying out of certain basic aspects of sewage digestion and which ledto the developmentof the present invention is illustrated in Figs. 2 to5 inclusive of said already issued Patent 1,989,589.

On that early work there was employed sludge agitating mechanism whichmay be operated intermittently or continuously as desired. Such earlymechanism comprised a rotatable vertical shaft preferably mounted offcenterin respect impeller was such that it effected a non-hori-w zontalflow or current movement, to wit, movements which are not at rightangles nor in a plane normal to the axis of the vertical shaft. Theother impellers spaced along the shaft were located in a horizontalposition and effect flow or current movements inplanes normal to theaxis of rotation of the vertical shaft. The function of the impellersthat were horizontally mounted, was to agitate the particles of thesludge in the tank in horizontal zones, while that of the top tiltedimpeller was to discourage if not to prevent the formation of scum inthe zone of the liquid level in the digester by razzle-dazzle orcircumnutatory flow or current movements which convectively carry scum.particles rising to and which would otherwise tend to float and collectat the top of the body of digesting sludge to submergence within thesludge whereby the scum particles ultimately become digested. In thisway the collecting and formation of scum was eliminated and avoided. Theeffect of this agitation with its consequent hindering of the formationof a floating scum layer and a deposited sludge layer or blanket was toensure that the entire sludge mass in the primary digester was broughtinto a relatively homogeneous condition by virtue of which portions ofsaid sludge mass withdrawn from the said digester for further ordewatering treatment have on the average substantially the sameproportion of liquid and solid constituents as said sludge mass.

In the apparatus of the early work primary and secondary sludgedigesters were employed and partially digested sludge from the primarydigester was suitably conveyed to the secondary digester or digestionzone or tank. This secondary tank was of the sedimentation type sincethe function of this tank was primarily to thicken the sludge and toobtain a clarified supernatant therefrom and secondarily to finish thebiological digestion of the sludge from the primary digester. The tank2? was therefore some times referred to as a sedimentation tank or basinfor in it there was maintained predominant essentially quiescentconditions as required for sedimentation without concurrentpredominating conditions conducive to digestion. Accordingly, digestingsludge from the primary digester Was fed to the secondary digesterthrough suitable piping.

In the early demonstration plant as to which the showing of Figs. 2 to 5of said Patent 1,989,589 is illustrative the primary tank was 29 ft. indiameter and 15 ft. deep. The impellers were of the Weedless type and 20inches in diameter. They were run 'at a speed of 120 R. P. M. althoughspeeds up to 600 R. P. M. may be used.

In the primary digester, the lower impellers caused horizontal rotationor agitation of the sludge mass to produce thorough mixing thereof inand throughout the horizontal section or stratum affected thereby. Theresulting turbulence causes an evolution of gas, which in rising to thetop of the digester, tends to form scum adjacent the liquid levelthereof. This tendency to form scum is particularly discouraged, if notovercome, by the action of a tilted impeller (typified by impeller 23 ofFig. 2) which set up a razzle-dazzle action or circumnutatory flowcurrent in the digesting sludge as indicated in Fig. 2, and serves tobeat back or diffuse the scum particles and any liquor which may rise tothe top of the digester into the digesting and horizontally agitatedmass or sludge.

Reference will now be made in detail to the later or other developmentswherein the novel yieldable and movable top constructions embodying thepresent invention in association with digester tanks are employed. Itwill be understood that for an' amplification of any part or descriptionwhich has been eliminated, foreshortened or amended that reference maybe made to the disclosure in the specification of Patent 1,989,589 sincethe disclosure in the application of this patent is the same as thedisclosure of the application Serial No. 728,376 from which said patenteventuated.

The construction of the associated digesters of Figs. 3 and 4 will nowbe described and more particularly in conjunction with reference toFigs. 5 to 11' inclusive. The tank N2 of primary digester H0 is made ofconcrete. It comprises a floor or bottom portion H6 and peripheralupstanding wall lll the latter of which carries at the upper interiorportion thereof an inwardly extending stop ring or corbel I09 providinga stop for the gas holder or gas holder construction H3 and forsupporting the latter in a lowermost position therefor at the upperinterior portion of the tank. This stop ring or corbel I09 serves toprevent the loss of gas through the liquid seal around the holder bydeflecting rising gas bubbles from the liquid sealing section. The tankH2 is also provided with a vertically extending centrally located guidepost 8 which is fixedly secured at H9 relative to the floor of the tankstructure.

The gas holder H3 is provided by a vertically yieldable and in fact avertically movable structure which comprises a sheet metal top l2!) anda depending peripheral wall l2l, a vertically extending centrallylocated strut Q22 and downwardly inclined connecting members I23 whichextend from the peripheral wall iii inwardly and downwardly to a placewhere they are connected to the vertically extending strut member H22.This vertically extending strut member I122 is preferably tubular inform and has a vertically sliding engagement with the guide post Mil.See in this connection the arrangement in Fig. 11 wherein the post M8 isshown as carrying anti-friction rollers M38 that engage an interiorsection of the strut or guide tube H22. This strut member i22 has a dualfunction, it serves in conjunction with guide post M8 to guide thevertical movement of the gas holder i l and to ensure that the top l2thereof shall maintain a desired horizontal arrangement for allpositions thereof. It also serves to reinforce and support the top 92dwhen the gas holder is resting in its lowermost position. This providesa construction for preventing the top lZt from buckling, bending orcollapsing incident to external air pressure exerted on the gas holder.

The top member or portion 52d of the gas holder is in the form of anumbrella cover or inverted saucer. More specifically and more ac=curately it may be described as preferably in the form of a top sectionof a hollow sphere, the periphery of which section is in plan a truecircle.

The gas holder however need not be circular in horizontal cross sectionas any other geometrical form which in horizontal cross section presentsa symmetrical arrangement could be employed as for example one showing ahexagonal, octagonal or other symmetrical form. Moreover the top memberor top construction of the gas holder need not be in the form of asphere top, of an inverted saucer, or resembling that of an umbrella topsince any suitable geometrical form could be employed, be it that of acone, of a pyramid, a frustrum of a cone or a frustrum of a pyramid. Anessential factor is that the top member or top construction of the gasholder shall be hollow so as to have a concave portion at the under sideand which is well constructed to withstand either internal or externalpressures as and when applied thereto.

The peripheral wall I of the top holder deof the latter.

pends downwardly from said concave top section or inverted saucer shapedtop member". This peripheral wall may be defined as a peripheral ring ormember which has a gas-tight connection or union with the top memberwhereby a gas-tight top construction for the holder is provided. Thisperipheral ring or wall lZl is of metal and is loaded with concreteballast IN. This concrete ballast is relatively heavy and the functionthereof is to place a relatively progressive and increasing load on thegas holder particularly as the gas holder rises sufliciently to lift theconcrete ballast so that it projects above the sealing liquid whereby aprogressively and substantially increasing load is imposed upon themovable holder according to the amount of ballast projected above theliquid level. In the lowermost position of the gas holder the lower edgeportion of the peripheral wall or ring iZl rests upon the stop ring orcorbel m9. An inspection of. the drawings will make it clear that thisstop ring is positioned so that the gas holder even in its lowermostposition is located adjacent to the upper portion of the primarydigester and also that the vertical movement of the gas holder duringany normal operation thereof is small or relatively limited particularlyas compared with the vertical movement of the gas holder for'thesecondary ently appear. As previously indicated, according to and withinthe broader aspects of the invention, it is feasible to construct theprimary digester so that the 'gas holder thereof can have a long rangeof movement and so that the gas holding capacity thereof can berelatively large.

The strut member I22 heretofore described as a tubular member isprovided with openings M6 at the top end thereof whereby any pocketingof gas that might otherwise affect the movement of the gas holder isavoided.

Mixing devices or sludge impelling mechanisms i243, frequently referredto as turbo-mixers, are carried by the gas holder so as to be verticallymovable therewith. The arrangement of these mixing devices is clearlyshown in Figs. 3 and 4 and the general construction thereof and mode ofmounting the same is clearly shown in Figs. 10 to 16 inclusive. Themixing device (or devices) Hi l is similar in essential operativecharacteristics to the mixing device or sludge impelling mechanismpreviously referred to in connection with the early system of Figs. 2 toinclusive of said Patent 1,989,589. Each of these mixing devices i2 5comprises a base H5 which is detachably secured to the top I20 asthrough the medium' of bolts I26 and in a manner whereby a gas-tightjoint is provided as between the top i213 and the base i255. Upon thisbase H5 there is carried the housing E21 of an electric motor I28 therotor of which is connected so to drive through the medium of suitablegearing and coupling ltd, a vertically extending shaft ltli of themixing device so as to effect the normal rotary movement Suitablegas-tight packing itl is provided below said coupling 8%. In theimmediate region of each mixing device a depending ring i32, which iscircumferentially arranged with respect to the shaft I130, depends fromthe cover a sufficient distance to extend into the liquid or digestingsludge maintained up to a minimum level within the primary digester whenthe latter is functioning. The purpose of this depending ring I32 andliquid seal resulting from the use thereof is to provide an arrangementwhereby, at will, the mixing device as a whole can be refor producingimpeller The inclination of the body as it digester, as will pres--moved from place without allowing the escape of gas from the holder bymerely removing nuts from the bolts I26.

Each mixing device is provided with non-clogging impellers as I33 invertically spaced arrangement. Each of these impellers comprises a hubsection We, a body section l35 arranged in an inclined position inrespect to the hub axis whereby when the impellers are in place thebodies are tilted at a slight angle from the horizontal and eachimpeller also includes vanes or blades H6 effects in directions awayfrom the axis. rotates produces vertical components of flow movementswhile the vanes or blades produce horizontal components of flowmovements and the functioning of these impellers produces what may bereferred to as the razzle-dazzle or circumnutatory motion or mixingaction within the liquid body afiected thereby. The functioning of theseimpellers, which located in respect to the general axis of the tank asis manifest from Figs. 3 and 4, produces what may be generally termedepicyclic type of currents or movements throughout the various sectionsof the liquid body.

Some or all of these impellers are frequently made and employed so thatthe body section thereof need not be inclined to the hub axis with theresult that the body section and the impellers move in true horizontalpaths thus avoiding most or much of the vertical components which areare also offset or eccentrically experienced where the body is inclinedto its axis.

Impellers with bodies at right angles to their axes are useful in thelowermost layers of the sewage sludge but it will be pointed out thatthe impellers with the bodies inclined to their axes of rotation areparticularly useful in the upper layers of the sludge since theoperation thereof assists in breaking down scum and in the thoroughmixing of the tank contents in such a manner that the accumulation ofscum at the upper interior portion of the primary digester is avoideddue to scum particles being swept by the razzle-dazzle currents back tosubmergence within the body of digesting sludge.

The mixing devices as illustrated herein have been described inconsiderable detail and as previously indicated function as non-cloggingimpellers or in other words are of the non-clogging type. It is feasibleto construct each impeller so that it primarily comprises or consists ofa body or hub portion and blade members, which body or hub portion issecured to the vertical extending propelling shaft to which the impellercorresponds and which blade members extend outwardly from said body orhub portion. Such blade members may be considered as compositely definedby the upper and lower blade elements heretofore described. The bladeelements function to effect a forced or circulatory movement of liquidaway from the shaft particularly since there is no peripheral restraintagainst outward flow because there is not any ring or other restrainingmember located at or in the immediate vicinity of the periphery of theimpeller.

As to the impellers herein shown some have body members in effectprovided by plates .that extend at right angles to the axis while somehave body members in effect provided by plates that are inclined to theaxis about which they are rotated. It is feasible, however, to constructimpellers that will function along the same lines as the impellersherein illustrated and. this even though the plate portions justreferred to are not necessarily employed. 18y bearing in mind theparticular work to be done by a particular impeller it is merely amatter of design to construct the impeller to do the work desiredtherefor.

The lower ends of the mixer shafts i139 are provided with removablesteady bearings each of which includes a bearing member 43? that isretained upon the shaft ldd and which is removable therewith because ofspaced collars l38-l3t which are secured to the shaft; In thisconnection see Figs. 10, 11, and 12. Each bearing member Ed? has anouter section N5 in the shape of a frustrum of a cone whereby it readilysets or positions itself in place on the conical seat sec= tion I353which is carried at the outer end of the corresponding extension armconstruction 1 39 that includes parallel channels Ml and M2. Theintermediate sections of these channels are secured in any suitablemanner M3 to the vertically extending strut member l22. Braces l l l arerelied upon for firmly supporting the outer ends of the arm constructionMd against downward deflection. It will be noted that the outsidediameter; of each impeller H33 and of the removable steady bearingmember Mill is less than the inner diameter of the depending ring I132with the results that the mixing device 92d as a whole can be readilyplaced at will solely by the operator removing 0 nuts from the bolts l2and vertically moving the of which extends into the mixing devices fromor towards its normal position in respect to the gas holder top.

The primary digester receives the sewage sludge to be digested in anysuitable manner as through the feed pipe M5 which it will be notedaccording to the arrangement shown delivers the incoming or influentsewage sludge into the upper interior portion of the primary digestertank, preferably through a section of the feed pipe M5 which is locatedsomewhat below and which extends past the supporting surface at the topof the stop ring or corbel me.

Sewage sludge, which has been treated, passes from the primary digesterthrough a discharge section M8 into a discharge pipe or conduit Ml!having a valved branch M8 and an tending branch M9 the latter of whichterminates in an adjustable overflow or weir construction it!) that isrelied upon for ensuring that the minimum level of the body of sewagesludge retained in the primary digester shall always be at least up tothe height determined by the overflow. For the details of the overflowarrangement of both the primary and secondary digesters see particularlyFigs. 5 to 8 inclusive.

In the normal functioning of the primary digester the valved branch M8is closed whereby all of the sludge passing from the primary digestermust pass through the overflow branch I49. It will also be here notedthat as sludge is supplied to the primary. digester for treatmenttherein a passes therefrom through the overflow branch I 49 on its wayto a secondary digester as will hereinafter more clearly appear.

The primary digester also has a gas oiftake piping or conduit I5I theupper intake end I52 gas receiving space which is always maintainedwithin the upper interior portion of the primary digester, to wit, intothe gas receiving space at the under side of the gas top I20 on the onehand and the top of the body of sewage sludge maintained within theprimary digester on the other hand. This gas pipas by welding atupwardly exing l5l extends into the secondary digester and. terminatesin a pipe section #153 the upper end of which extends into a gas spaceprovided in the upper portions of the secondary digester with the resultthat the digesters the gas spaces of both digesters are in constantcommunication. The upwardly extending delivery end portion or pipesection I53 derives support from the floor of the tank of the secondarydigester througlr the medium of supporting rod l53'.

The primary digester is also preferably provided with .a normally closeddepending pipe 554 (see Fig. 3) that always dips into the sludge withinthe primary digester but which is arranged to provide a sampling openingby which ready access for sampling purposes is realized.

The primary digester is provided with heating coils E55 which are usefulin ensuring sufficient heat to the sewage sludge to facilitate'thedigestion that is carried out therein. The gas holder is secured againsthorizontal rotationrelative to the tank through the medium of chains'556 and which are referred to as tangential guides. T

The secondary digester ill is shown larger than the primary digester andcomprises a sec- -ondary digester or sedimentation tank Ht providedwitha downwardly and inwardly inclined bottom Me and upstandingperipheral wall ill carrying spaced stops Mill. The secondary di- Theperipheral the latter is in I M6 for the secondary digesterand inwardlyand ly located strut corresponding thereto. The

lower edge portions of the peripheral ring ltd are 50 construction thesecondary gas holder is guided throughout the entire range of its normalvertical movement and the strut construction also v serves to interiorlysupport the plate top section of the digester against internal bucklingor collapsing should there be realized a decrease in pressure within thetank and an excessive unbalanced pressure on the outsideof the tank.

Reference has heretofore been made to the valved branch I48 of theprimary digester.

sewage solids disposal as along the pathSB to a sludge bed. The tank ofthe secondary digester has a solids 75 during the normal functioning of5 I10 leading to a valved sludge pipe or conduit "I which terminates ina T pipe fitting I69. It will therefore be apparent that the pipingwhich includes the pipe I41, cross fitting I69 and pipe I1I when thevalves are open can be employed for the direct passage of sewage sludgefrom the primary digester to the secondary digester. Normally the valvesare closed so this direct transfer cannot take place. Upon the openingof either the valved pipe I41 or the valved pipe I1I, it will -bemanifest that sludge can be pumped at will from the primary digester orsecondary digester to and through the cross fitting I69, thence alongthe path I90 by a pump as 1 or 8. See Figs. 6, 4, 3, and 1. It wouldalso be possible to effect a reverse pumping action to unplug the valvedpipes I01 or I1I should occasion require. The secondary digesternormally functions over an extended period without the withdrawing ofsludge and then at the proper time the deposited solids remaining afterthe digestion and sedimentation carried out therein are removed throughthe pipe III. This removal of sludge solids of course effects a loweringof the normal water level within the secondary digester and this isaccompanied by a lowering of the gas holder. In order to avoid loweringof the liquid within the secondary digester in such a manner as tounduly reduce the gas pressure therein whereby too great a differentialas between the exterior air pressure and the internal pressure will takeplace-so great a differential as will tend to cause the gas holder tocollapse or fail-there are provided certain ordinary check arrangementsas follows: a sludge check or ball valve I12 that is carried by chainsI13 is provided and which sets itself in the solids discharge sectionI10 slightly before or .at least by the time that the secondary gasholder reaches its lowermost position; also a gas check comprising avalve or pipe closing member I19 supported at the upper end of rod I'I5arranged to effect a closing of the upper end of the gas pipe or conduitI53 at or by the time the secondary gas holder reaches its lowermostposition with the result that communication between the digesters isshut off and also with the result that further withdrawal of gas fromthe secondary digester cannot take place. A sealing ring I60 dependsfrom the top member I53 of the gas holder for the secondary digester(see Fig. This sealing ring dips into the sludge liquid within thesecondary digester when the gas holder is in its lowermost position andprovided the liquid is up to the minimum height as determined by theoverflow weir construction for the secondary digester. Under suchconditions access to the valve I14 is afforded without all-owing theescape of gas from within the secondary gas holder. Likewise a similarsealing ring I59 is provided at the under side of the top portion of thegas holder for the primary digester. The sealing ring I59 dips into thesewage sludge when the gas holder is in its lowermost position. Underthe conditions specified-with the gas holders in their lowestposition-the flow of gas therefrom into the gas pipe I5I is prevented.

It hasheretofore been pointed out how the sewage which has undergonetreatment in the primary digester passes therefrom by the overflow I50as ,sewage sludge to be treated is supplied to the primary digesterthrough the feed pipe I45. I

An inspection of Figs. 6 to 8 inclusive will make it clear that thedigesting sewage sludge which discharge section rangements passes theoverflow I50, enters the receptacle or trough I16 and flows therefromthrough a pipe I11 that delivers the sewage to the secondary digesterand more particularly into the lower section of the liquid bodymaintained within I the secondary digester.

These figures also show a second overflow pipe or conduit I18 leadingfrom the primary digester and terminating in the adjustable overflowsection I19 that is somewhat higher than the overflow I50. This secondoverflow: I18 and I19, is provided in case the overflow branch I49 whichterminates in overflow I50 becomes plugged or unduly loaded, therebyavoiding any undue excess of liquid level within the primary digesterduring the normal operation thereof.

The secondary digester has an overflow construction relatively remotefrom the place where sewage sludge is introduced into this secondarydigester as will be seen from an inspection of Figs. 3 to inclusive. Theoverflow arrangement for the secondary digester comprises an offtake oroutflow pipe or conduit I80 leading from below but near the upper normalwater level for the body of liquid within the secondary digester as isdetermined by the overflow arrangement for this digester. This pipe orconduit I80 leads through the wall of the digester and termimates in anadjustable overflow or weir section IBI for determining the maximumnormal water level within the digester. The upwardly extending intakeend section of the pipe or conduit I80 derives support from the floor ofthe tank of the secondary digester through the medium of supporting rodI80. In case this pipe construction I80 or I8I should become plugged orineffective there is provided an auxiliary overflow or outflow pipe I82that terminates in an adjust able overflow or weir I83 the latter ofwhich is somewhat higher than the overflow I8I. For convenience andcomparison the overflow arof the primary and secondary digesters havebeen brought into close relationship in Figs. 5 and 6. Overflows I8I andI83 of the secondary digester deliver into a tank or receptacle I84 fromthe bottom of which there extends a pipe or conduit I05 for conductingthe relatively clear delivered effluent to such place of use or disposalas is deemed desirable. The normally functioning overflow I50 for theprimary digester occupies a higher elevation than the normaloverflow IOIor even the overflow I83 for the secondary digester and it will beapparent that when the liquid level within the secondary digester is upto the normal maximum level as determined by overflow I8I that a feed ofsewage into the primary digester simultaneously effects a flow of acorresponding amount of treated sewage from the primary digester intothe secondary digester and a corresponding flow of relatively clearefiiuent from the secondary digester past the overflow I8I thereof. Thesecondary digester is preferably provided with sampling pipes I86, IBI,etc., where samples from the various levels within the secondarydigester can be readily obtained. The secondary digester has anysuitable sway bracing and guides such as are provided for example bysway chains I98 and I89 and by the vertical guideways indicated at I90.

In certain localities where freezing may take place arrangement is madefor employing a reprimary digester or at 94 for the secondary digester.

The combination of primary and secondary dgesters as just described indetail particularly as shown in section in Fig. 3 typifies theconstruction of the primary and secondary digesters employed in thesystem of Fig. 1. A somewhat which appears in As previously stated thesewage supply to be treated passes from SS along a path l or 3 as thecase may be, to a clarifier or thickener 2 or d wherein the sewagesludge is settled out and from which the sewage sludge thus settled outis withof either the raw sewage prior to sedimentation in the clarifier2 or therefrom can be carried out.

The efiluent from the clarifier is passed therefrom as along the path 2Aor 4A for such disposition as may be desired for a particular plant. Anyscum collecting at the top of the clarifier may be passed therefrom asalong the path 23 or 4B to a point where it will join the sludge fromthe clarifier 1 Sludge pumps 7 and 8 are connected in the piping systemthat includes the pipe lines 5 and 6 whereby at will either one of thepumps can be operated to function in the pumping of sludge from eitherof the clarifiers or whereby if desired one pump can be pumping from oneclarifier while u the other is pumping from. the other clarifier orwhereby one of the pumps may be pumping from one clarifier while theother of the pumps may be pumping sludge solids from the secondarydigester as will hereinafter more fully appear.

Either or both of the pumps '1 and 8 can be relied upon as desired topass sewage sludge into the primary digester through the feed pipesection I45 thereof or at will either or both of the pumps can deliverliquid pumped thereby along the path 9B Inthe normal functioning of thesystem this line I00 is closed and the normal functioning of the pump orpumps then is to deliver raw sewage 75 sludge into the primary digesterthereby forcing indicates a valve 7 4 or of the sewage sludge digesterand which cross member 569. The gas piping or conduit which includes theparts E52, 95! and I53 previously referred to (and in this connectionsee Figs. 1, 3, 4, and 6) places thegas receiving portions of theprimary and The gas will be employed for heating by passing the samealong the path 504 through condensate trap CT past meter water boiler205 or dihot water boiler includes a heating coil H0 which is in acirculatory system that includes the heating coils l 55 of the primarymay also include the removthe sheet metal top buoyant peripheral or roofportion 2|3 thereof a ring provided by the hollow vated positionapproximately that the dotted lines.

- to the arrangements shown and tions depending from struction providedby the hollow annular section 2 described is sufficiently heavy toeffect a material loading of the topwhereby any extended liftingmovement of the and incident to fluctuation in gas pressures duringnormal operation is avoided.

In Fig. 15 there'are shown a primary digester and secondary digestersimilar 'in many respects described in conjunction with Figs. 3 and 4.According to the construction of Fig. '15, however, the gas holder ofthe primary digester is vertically yieldable but it is loaded so as toremain during the normal operation of the digester in the lowermostposition therefor, to wit. at rest upon the stop ring or corbel I09. Inthis figure the gas holder as a whole is designated as 3"). It comprisesthe umbrella shaped sheet metal top member or roof portion 3H! and has aperipheral ring 3", part of which ring depends from extends above theperipheral edge of the umbrella shaped portion 3l3. At the exterior topperipheral portion of this gas holder or top member 3"], as at 3H3,there is placed suitable weighting material as concrete which ensuresweight somewhat in excess of that required for normally holding thecover in lowermost position on the stops and which functions inopposition to any buoyant effect of the sludge liquid within the tank H1and against the normal gas pressures experienced within the digester.Otherwise than as indicated the arrangement of this Fig. 15 operatessubstantially along the lines described in connection with thearrangement of Figs. 3 and 4.

We claim:

1. A gas holder construction for use in conjunction withv a sewagesludge digesting tank comprising in combination atop member havinintermediate the central section and the periphery thereof aperturedsections for receiving agitating means, a depending portion providing aperipheral flange having a gas-tight connection with the top member,intermediate flange porthe'intermediate apertured sections and havinggas-tight connection with said sections, a vertical strut memberextending downwardly from the central section of the top member,connecting members extending downwardly from the region of saidperipheral flange by said arms in the regions .ceiving and holding tothe lower end of said strut member, for. each of said apertured sectionsan agitating means comprising a vertically extending thereon and aseating member at the'lower end thereof, arms outwardly extending fromand carried by said strut, and seating members carried directly belowthe apertures of said apertured sections, which last mentioned seatingmembers are provided for rethe corresponding seating members oftheagitating means.

2. A gas holder construction as defined in and by claim 1 according towhich the intermediate flange portions extend downwardly to an elevationwhich is lower than that of the lowermost edge portions of theperipheral flange.

3. A gas holder construction as defined in and by claim 1 according towhich a hollow gas-tight annular pontoon is provided which includes thedepending peripheral flange as a portion thereof. 4. A gas holderconstruction as defined in and by claim .1 according to which an annularcontop relative to the liquid and part of which ring gal type,

shaft, impellers I aperture.

' by claim 1 according to which the agitating means are removable andreplaceable by vertical movement, according to which the seating memberon each arm and the seating member carried by the shaft of thecorresponding agitating means are constructed so as to provide conicalseating surfaces for self-adjustment and automatic alignment of theassociated shaft as introduced in place by vertical movement, and alsoaccording to which the seating member carried by its shaft is mounted sothat vertical relative movement between it and its shaft can take placeas the shaft is moved downwardly and incident to the last mentionedseating member coming into operative seating engagement with thecorresponding seating member carried by the arms.

8. A construction as defined in and by claim 1 according to which theagitating means comprises a motor, according to which the verticallyextending shaft is actuated through the medium of speed reducing gearingoperatively located between therotor of the motor on the one hand andthe shaft on the other hand, according to which the shaft has thereuponat different elevations along the same impelling devices of thecentrifuand according to which the seating member on the shaft ismounted so as to permit not only rotary movement but also verticalmovement as between the shaft and the seating member.

9. A gas holder construction for use in conjunction with a sewage sludgedigesting tank comprising in combination a sludge stirring device; asheet metal top member having intermediate the central section and theperiphery thereof an intermediate aperture for receiving the sludgestirring device, 9. depending portion providing a peripheral flangehaving a gas-tight connection with the top member and an intermediateflange portion depending from the section defining the intermediateaperture and having gas-tight con nection with said section; a verticalstrut member extending downwardly from the central'section of the topmember; connecting members extending downwardly and inwardly from theregion of said peripheral flange to the lower end portion of said strutmember; said sludge stirring device comprising a vertically extendingshaft driven from above the top member depending downwardly through theintermediate aperture and carrying sludge impellers located at theunderside of the sheet metal top member; an arm construction providingan arm extending from said strut;,and a steady bearing for andassociated with the lower end portion of the shaft and carried by saidarm in the region directly'below the intermediate ANTHONY J. FISCHER.inns B. LUND.

to which the strut is a cen-

